While the invention is subject to a wide range of applications, it is especially suited for use in an interferometric system for testing a lens and will be particularly described in that connection.
The quality of a lens is primarily determined by the shape of its point spread function (PSF) or its optical transfer function (OTF). At present, direct measurements of the PSF are carried out by either photographic or electromechanical methods. These include a slit or pin hole scanning of the focal plane. Either of these methods allows the measurement of the square of the modulus of the electric field of the image and leaves its phase unknown. In order to determine the phase, a two-dimensional Fourier Transform is performed on the optical path difference (OPD) at the pupil of the lens to be tested. This OPD is measured by interferometric methods. In the past, several different techniques were used for evaluating the quality of a lens. For example, clasically PSF measurements were made by obtaining the phase and then the OPD at the pupil. This was a rather complicated, time consuming procedure. The present invention requires less experimental time due to multiplexing and provides a simplified mechanical design without slits and a minimum of movable parts. Also, the present invention provides for easy computer interface.
Another popular technique for evaluating lenses uses interferometric methods as disclosed in U.S. Pat. No. 3,936,160 to Von Bieren which discloses for example "An interferometer method for measuring aberrations in a lens system by analysis of a Fourier Transform pattern generated by the lens system". Another use of interferometric measurements is disclosed in U.S. Pat. No. 3,912,394 to Keisall which discloses for example a method and system of interferometric measurements of modulation transfer function.
The present invention provides significant improvements over the prior interferometric methods. These include full elimination of two .pi. uncertainity on the phase, which allows for complete mapping of the wave front. The mapping is possible even when strong distortions are present such as in the case of inexpensive lenses or aspheric elements. Further, the invention is less expensive than the prior art because no Vidicon is involved. The electronic interface is much less complicated and the measurements at all of the wavelengths are made by simply changing a source and a single detector. Thus, a scanning detector is unnecessary. Problems which might arise from diffraction at the borders of the pupil are eliminated. The present device provides easy computer interfacing. The need for overdimensional interferometers is eliminated as opposed to the prior known shear method. The calculation of MTF, the Fourier Transform of (complex PSF).sup.2, is more difficult than to calculate the autocorrelation of the OPD. Finally, no holograms are required with the present method.
The present method has some definite advantages over the MTF measurements which are the periodical-noise correlation pattern imaging. The present method includes the correlation of the OPD which eliminates any problems of orientation of phase. Also, the time of experimentation is reduced due to the multiplexing. The equipment for performing the present calculations is of a simplified mechanical design. Further, there are no errors due to the finite object size. In addition, errors do not occur due to nonsinusoidal or nonlinear patterns. Also, no photographic processes are involved and no problems of noise correlation.
In the present invention, the technique is a multiplexing method for measuring the PSF. In other words, all of the light from the lens to be tested arrives on the detector instead of a small fraction of it as with the traditional methods using slits or pinholes. The consequence is a much faster recording time. Also, virtually unlimited resolution can be achieved which is not generally the case of the traditional methods since they obtain a convolution of the PSF with the transparency function of the slit or pinhole.
Further, as mentioned above, the OPD, OTF and MTF can be obtained by performing a two dimensional Fourier Transform on the measured CPSF or the square of its modulus, the PSF, on a digital computer. The method of measuring these quantities can be thought of as a Fourier Transform multiplexing process in the sense that the light from the whole area of the pupil is received by the detector at the same time. Here also, the time and measurement and resolution can be improved by orders of magnitude as the limiting factor is the time of computation of the two dimensional Fourier Function Transform. In addition, the present invention overcomes the problem of reducing the interferograms obtained by the basic Twyman-Green interferometer to an OPD map. Also, another advantage of the present invention is the ability to test rough surfaces.
It is an object of the present invention to provide a method and apparatus of measuring the quality of a lens.
It is an additional object of the present invention to provide an interferometric system which can very quickly determine the quality of a lens.
It is a still further object of the present invention to provide unlimited resolution of the lens to be tested.
It is a still further object of the present invention to provide an interferometric system which can measure many different types of lenses.